Biometric systems enroll, verify, and identify subjects by acquiring and comparing biometric samples from a subject. A biometric capture device is responsible for the acquisition portion of the system. The boundary of space within which a sample may be acquired by the device is defined as the capture volume. A capture volume may vary in number of dimensions, shape, size, and other attributes. When the biometric capture device uses a camera to acquire samples the capture volume is generally defined as a frustum shape in three dimensions. Facing the sensor the subject can move left or right (X-dimension), squat down or stand up (Y-dimension), and move towards or away from the sensor (Z-dimension). Moving outside the X or Y limits of the sensor's field of view prevents the sensor from imaging the subject. The limits in the Z dimension are more complicated. As the subject approaches or retreats from the sensor several physical limits start to impact the acquisition system. For example, the system may be nominally focused for a given distance. Moving away from that optimal focus point, the acquired samples will begin to get blurry which is generally unacceptable. Additionally, the sampling density of pixels on the subject varies with distance due to the expanding nature of the imaging frustum. This assumes the user is still able to align in X and Y as the size of the frustum in space shrinks proximal to the sensor. In the case of an iris biometric capture device, infrared illuminators may be used to provide an appropriate light level over the intended capture volume. When the subject is positioned outside the intended capture volume, the light level may be too intense or too weak to acquire an acceptable biometric sample. Therefore, any system must provide a method for a subject to align in X, Y, and Z to acquire an acceptable biometric sample.
One existing alignment method is to show the subject the output from the capture sensor which is effectively an electronic mirror. A simple on-screen indicator such as a box may be used to show the extents of the X and Y capture volume dimensions. The electronic mirror method provides intuitive and rapid feedback for alignment in the X and Y dimensions, but fails to provide adequate feedback in the Z-dimension since the subject will continue to be visible regardless of their distance. Therefore, additional feedback must be provided to guide the user to an appropriate distance. Another existing alignment method is by using text. Text can be added to the electronic mirror feedback indicating “move closer” or “move away” to help guide the user, but this requires they shift their attention to the text, read it assuming they understand that language, and follow the guidance.
One method used to provide Z-dimension feedback is audio feedback instructing the subject to move closer or further. Using audio is a relatively slow and imprecise feedback method. Words are slow to enunciate and take time for the subject to interpret their meaning before being able to start moving. Simple feedback such as “move closer” does not provide any indication of how far to move forward. Precise feedback such as “move forward 2 inches” is difficult for most subjects to accurately move. Additionally, the subject may have moved some distance while the words were being spoken rendering the late feedback inaccurate. This of course assumes the subject knows the language being spoken, the environment is quiet enough to hear the prompts, the voice prompts are not disturbing others, or the like.
Using sounds instead of spoken words can speed up the feedback loop. An example of this is using faster/slower or higher/lower pitch beeps to indicate relative distance. This method works well for providing distance feedback on one side of a capture volume. This type of technique is used to assist drivers backing up a car to avoid hitting an obstacle; it isn't intended to provide feedback on either side of the obstacle. This method must be modified to provide direction indication as well as distance from the capture volume to provide sufficient information to guide a subject into the capture volume.
Another method used to provide Z-dimension feedback is on-screen visual alignment aids. Visual aids may be biometric alignment aids or distance indicators. An example biometric alignment aid could be a pair of circles where the subject is supposed to align both of their eyes. Due to variations in human anatomy, biometric alignment aids can only provide relatively coarse distance feedback. More accurate positional feedback can be provided with a distance indicator. A range meter can simultaneously show which side of the capture volume the subject is on, relative distance to move, and the size of the volume to get within. This complicates the user experience by requiring the subject to alternate their focus between the electronic mirror images and the distance feedback to achieve alignment in X+Y and Z respectively.
Thus, a need exists for an improved method of providing feedback to users so that they may correctly align themselves in all dimensions quickly and accurately. This and other needs are addressed by the systems and methods of multidimensional alignment aid of the present disclosure.